The Radiological Physics Center (RPC) was established in 1968 upon the recommendation of the Committee on Radiation Therapy Studies (CRTS), under the sponsorship of the American Association of Physicists in Medicine (AAPM) to insure the correctness and consistency of radiation dosimetry among institutions involved in interinstitutional clinical trials. This is accomplished ultimately by a review of the radiation therapy treatment records of patients entered onto interinstitutional clinical trials to verify that the treatment reported was in fact delivered and was in accord with the specifications of the clinical trial protocol. The RPC presently reviews the dosimetry aspects of individual patients' treatment records for patients entered on protocols of 6 clinical trial groups. The RPC also provides dosimetry data to 5 radiotherapy quality audit offices who review individual treatment records for patients entered on protocols of 10 clinical trial groups. The basic dosimetry data used in these reviews is obtained by a dosimetry review visit by an RPC physicist to the participating institutions. Mailed dosimeters are evaluated on a periodic basis to identify potential errors and to assure the RPC that the institution has not modified its dosimetry practices since the last dosimetry review visit. When errors are discovered the RPC works with the institution to rectify the errors. Efforts are made to develop calculative methods and efficient techniques for measurements that are important to clinical trials. The RPC provides advice to the cooperative groups on quality assurance questions including assistance in drafting the radiotherapy portion of protocols. Experience gained in developing quality control procedures for one clinical trial is communicated to other clinical trial groups and to the radiological community. The RPC provides this quality assurance monitoring to all 14 cooperative groups presently funded by NCI and those groups that are presently phasing out their studies. Dosimetry monitoring is provided for external beam photon and electron therapy for intracavitary and interstitial brachytherapy, and fast neutron therapy. There are 696 active U.S. institutions and 45 active in Canada and other foreign countries.